The present disclosure relates to a method and corresponding apparatus for transferring data according to an ARQ method, especially a hybrid ARQ method, in a communication system, such as a mobile radio system, for example
The use of so-called “packet-access methods” or “packet-oriented data connections” is often recommended especially in connection with mobile radio systems, since the message types produced often have a very high burst factor with the result that only short periods of activity exist, interrupted by long breaks. Packet-oriented data connections may, in this case, considerably increase efficiency compared to other data transfer methods in which a continuous data stream is. This is because in data transfer methods with a continuous data stream, once a resource has been allocated, such as, for example, a carrier frequency or a time slot, it remains allocated during the entire communication relationship (i.e., a resource remains occupied even if there are momentarily no data transfers taking place, which means that this resource is not available for other network users). The result is that the narrow frequency range available for mobile radio systems is not used to the best effect.
Future mobile radio systems, such as, those that comply with the mobile radio standard UMTS (Universal Mobile Telecommunications System), for example, will offer a multitude of different services whereby multimedia applications will become increasingly prevalent alongside pure voice transmission. The diversity of services associated with this, with different transmission rates, requires a highly flexible access protocol on the air interface of future mobile radio systems. Packet-oriented data transmission systems have proved to be highly suitable in this context.
In connection with UMTS mobile radio systems, a so-called ARQ (Automatic Repeat Request) method has been proposed in packet-oriented data connections. In this method the data packets transferred from a transmitter to a receiver are checked for quality at the receiving end following decoding. If a data packet is erroneous on receipt, the receiver requests retransmission of this data packet by the transmitter (i.e., a repeat data packet that is identical or partially identical to the one previously sent and which was erroneous on receipt, is sent from the transmitter to the receiver (the terms full or partial repeat are used to indicate whether the quantity of data contained in the repeat data packet is less than or equal to that of the original data packet)). With regard to this ARQ method proposed for the UMTS mobile radio standard, which is also known as a hybrid ARQ method, the intention is for both data and so-called header information to be transmitted in a data packet, whereby the header information also contains information for error checking, (e.g., CRC (Cyclic Redundancy Check) bits), and can also be coded for error correction (known as FEC, Forward Error Correction).
In accordance with the current status of UMTS standardization, it is proposed that the bits in the individual data packets and/or repeat data packets be transferred following execution of a corresponding channel coding by means of QAM modulation (quadrature amplitude modulation). In this procedure the individual bits are mapped, via a process known as “gray mapping”, onto corresponding QAM symbols which form a two-dimensional symbol area. This is problematic, since, in the proposed QAM modulation with alphabetic scope, which includes more than four QAM symbols, the reliability of the bits to be transferred varies considerably between the higher-value bits and the lower value bits. This is particularly disadvantageous with regard to the channel coding that is to be carried out, -since, for this-purpose, it is preferable to use turbocoders which require the reliability of the bits to remain consistent in order to achieve a sufficiently high level of efficiency. In a hybrid ARQ method, in which the repeat data packet is identical to the original data packet, the result of the aforementioned feature of variable bit reliability is that certain bits of the data packet and repeat data packet are to be found at the same place in the QAM symbol area, thus reducing the efficiency of the entire data transfer and limiting the data throughput at an early stage.
In order to resolve this problem it has previously been proposed that those bits which occur in the same place in the original data packet and in the repeat data packets be mapped to different QAM symbols in the QAM symbol area by dynamic rearrangement of the “gray mapping”.
This will be explained in greater detail below with reference to FIGS. 4A-4D. FIG. 4A shows the signal constellation/QAM symbol area for a 16-QAM modulation, in which bits i1 and i2 as well as q1 and q2 are mapped to a corresponding QAM symbol 26 of the two-dimensional QAM symbol area 25 in the sequence i1 q1 i2 q2. Each of the columns/rows of QAM symbol 26 in the two-dimensional QAM symbol area 25 that can be used for each bit i1, i2, q2, q2 is marked with a line. Thus, for example, the bit i1=“1” can only be mapped onto QAM symbols in the first two columns of the QAM symbol area. Thanks to “gray mapping” the reliability of the higher-value bit i1 is greater than the reliability of the lower value bit i2. In addition, the bit reliability of the bit i2 fluctuates depending on the corresponding QAM symbol 26 transferred (i.e., depending on whether the corresponding QAM symbol 26 is arranged in the outer left or outer right column of the QAM symbol area 25). The same applies for bits q1 and q2, since bits q1 and q2 are mapped in a manner equivalent to the mapping of bits i1 and i2 (albeit orthogonally for this purpose).
According to the conventional methods explained on the basis of FIGS. 4A-4D, it has been proposed that a different “gray mapping” be used for repeat data packets than the one used for the original data packets. Thus, for example, the “gray mapping” illustrated in FIG. 4B can be used for a first repeat data packet, while a “gray mapping” as shown in FIG. 4C is used for a second repeat data packet, and a “gray mapping” as shown in FIG. 4D can be used for a third repeat data packet. Comparison of FIGS. 4A-4D clearly shows that different QAM symbols 26, i.e., different points in the two-dimensional QAM symbol area 25, are mapped to one and the same bit combination i1 q1 i2 q2. This dynamic variation of the “gray mapping” may, for example, continue to the extent that, after a certain number of repeats, each bit i1, i2, q1 and q2 is transferred to a place in the QAM symbol area 25 with excellent or good reliability or poor reliability, whereby this procedure can be optimized for a different number of repeats.
It may be seen from FIGS. 4A-4D that this procedure is relatively costly since the “gray mapping” process must be modified for each repeat data packet.